Thursday, 30 July 2015

Babibasiliscus alxi: A Casquehead Lizard from the Early Eocene of Wyoming.

Casquehead Lizards, Corytophanidae,
are a group of Iguanid Lizards found from the tropical forests of southern Mexico,
through Central America to the northwest of South America. They belong to a
group of Iguanians, the Pleurodonta, found today almost exclusively in the
Americas, with non-American members of the group known only from the islands of
Madagascar and Fiji, and studies of the group taking into account relationships
between modern species and their biogeographical distribution suggest that they
probably originated in the American tropics. However a single fossil specimen
of a Casquehead Lizard is known from the Early Eocene of Central Europe,
suggesting that their ancient distribution might have been somewhat different
from that seen today.

The new species is named Babibasiliscus alxi, where ‘Babi-’ means ‘older male cousin’ in the
language of the Shoshone people who inhabited the region where the fossil was
discovered, and ‘-basiliscus’ (Basiliscus) is the name of a modern
genus of Casquehead Lizards, while ‘alxi’
honours John Alexander, who discovered the specimen from which the species was
described. The single known specimen comprises a nearly complete skull with
articulated jaws plus the first two vertebrae, undistorted or compacted, but
with the premaxilla, posterior half of the frontal, parietal, postfrontals,
postorbitals, squamosals, supratemporals, supraoccipital, dorsal half of each
quadrate, and the anterior tips of both dentaries lost to erosion.

Babibasiliscus alxi:
Photographs in (A) right lateral, (B) dorsal, and (C) ventral views. Digital
reconstructions derived from High Resolution X-ray Computerised Tomography in
(D) left lateral view and (E) transverse section. The vertical red line in (D)
indicates the plane of section in (E). Conrad (2015).

A phylogenetic analysis of
Casquehead Lizards including Babibasiliscus
alxi places it firmly within the modern group, closely related to the
living genus Laemanctus. Wyoming is
considerably outside the range of modern Casquehead Lizards, with a strongly
seasonal climate, typically including heavy winter snowfall. However during the
Eocene global climates are known to have been considerably warmer than today,
and both Babibasiliscus alxi from
Wyoming and Geiseltaliellus maarius
from the Eocene of Central Europe are interpreted as having lived in tropical
forests not dissimilar from those inhabited by modern Casquehead Lizards.

Phylogeny of Corytophanids and distribution of the group. (A)
Time-calibrated phylogeny of the Corytophanid genera with representative
illustrations of their skull morphologies. Conrad (2015).

Modern Iguanians are divided into
two groups, the Chamaeleontiformes, comprising the Chameleons, Agamas, and
their relatives, which are found in the Old World, and the Pleurodonta,
comprising the Iguanas, Basilisks, Anoles, Fence Lizards, and their relatives,
which are found in the Americas (with the exception of the Opluridae which are
found in Madagascar and the genus Brachylophus
from Fiji). However the earliest Iguanians are found in Cretaceous deposits
dating to before the separation of North America and Eurasia, with the majority
of species found in Asia, suggesting this is where the group originated. By the
Eocene Iguanians were divided into the modern groups Chamaeleontiformes and Pleurodonta,
though with specimens from both groups found on both sides of the Atlantic, and
in particular many species of Pleurodonts found in the Old World. It is
therefore thought that the modern distribution of the groups reflects a relict
distribution shaped by post-Eocene cooling rather than the modern groups having
derived from isolated populations after the separation of the continents.
Conrad suggests that this interpretation can also be applied to the modern
Casquehead Lizards, with the group distributed across tropical forests in what
is now the Northern Hemisphere temperate zone during the Eocene (when they were
probably excluded from the far hotter tropics), and subsequently having
withdrawn to the Neotropics during later Cainozoic cooling.

Anles are small Iguanid Lizards found in the Southern United States, Mexico, Central America and the Caribbean. Twig Anoles, Norops spp., form a distinct cluster of medium sized Anole Lizard species within the genus Norops. There are currently eight...

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About Me

Studied Palaeobiology & Evolution at the University of Portsmouth, Geosciences via the Open University & Ecology and Conservation at Christchurch University, Canterbury.
Have worked in wildlife based tourism, mineral exploration, development, conservation, education & environmental chemistry. Occasionally write articles for papers and magazines.

This Blog would be impossible without the work of countless scientists (and others) throughout the world. Where possible I do my best to credit them, but there will always be many more who remain unmentioned; this does not imply I am ungrateful for their contributions. Any errors or inaccuracies are, of course, my own.